Musnad Ahmad 2:65

Abu Sa’id narrated to us, Ibn Lahi’ah narrated to us, he said: I heard ‘Ata’ ibn Dinar, from Abu Yazid al-Khawlani, that he heard Fudalah ibn ‘Ubayd saying: I heard ‘Umar ibn al-Khattab that he heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "The martyrs are three: A believing man of good faith who met the enemy and was truthful to Allah until he was killed; that is the one to whom people will raise their necks on the Day of Resurrection—and the Messenger of Allah (peace and blessings of Allah be upon him) raised his head until his cap or the cap of ‘Umar fell off. And a believing man of good faith who met the enemy, and it was as if his skin was being struck by the thorns of the acacia tree, then a stray arrow came to him and killed him; he is in the second rank. And a believing man of good faith who mixed a righteous deed with another that was evil, met the enemy and was truthful to Allah until he was killed; that is in the third rank."